The embryonic and early post-natal mammalian CNS dynamically changes in cell number and anatomical structure as cells proliferate, undergo apoptosis, migrate and differentiate. This relentless change poses a formidable challenge to elucidate biologically-relevant processes among pre- and post-mitotic elements that help to explain the complex process of morphogenesis. Despite this, new levels of understanding with long-lasting implications were achieved during FY97. We used flow cytometry, micro dissection, buoyant density fractionation, dissociated cell culture, microchemotaxis, and immunocytochemistry to study the cellular distribution of biological properties deemed important in morphogenesis, including transmitter/receptor expression(s) and functions in the embryonic/early postnatal rat CNS. Studies were also started in the embryonic mouse, which can be compared to the rat and serve as a baseline in investigating transgenic models or genetic strains with CNS pathologies, and in the CP/SP neurons and progenitor/precursor cells in the VZ/SVZ. Immunocytochemical study of different components of putative GABAergic signalling pathways, including GAD65, GAD67, GABA, and GABA-A receptor subunits, has been carried out in conjunction with flow cytometry to quantify component expression/co-expression among the ganglioside-identified subpopulations. Most of the GABAergic components are well expressed among cells identified as neuronal in lineage. The majority of CP/SP neurons and subpopulations of post-mitotic neurons in the VZ/SVZ express GAD and GABA and various GABA-A receptor subunits. The distribution of immunodetectable GABA is quite labile, disappearing within minutes when live cells in suspension are briefly exposed to experimental conditions that stimulate neurotransmitter release, before processing for GABA-immunocytochemistry. Potentiometric and Ca-indicator dye recordings of entire intact cortical subpopulations have been carried out to outline the distribution of functional receptors for growth factors, peptides and all the major fast-acting transmitter families and specific ion channels. Clear patterns in the cellular distributions of functional receptors and ion channels have been detected among subpopulations. This unique perspective not only provides a completely new and unparalleled level of resolution into the cellular distribution of specific surface receptors and cytoplasmic Ca (cCa) signals in the context of morphogenic stages, but it also permits prospective investigation of cellular and molecular details in vitro since cells can be sorted based either on ganglioside expression/co-expression and/or functional receptor or ion channel expression. Fluorescent surface markers commonly used to identify cells undergoing apoptosis (like annexin V) have been applied in conjunction with flow cytometry to reveal the developmental distribution of putatively apoptotic cells during corticoneogenesis in the context of proliferative cells and differentiating lineages. Surprisingly, about half of the early embryonic cells actively in proliferation appear to be in four recognizable stages of apoptosis, as indexed over a 1000-fold range of annexin-V fluorescence signal intensity. The lowest levels of intensity are expressed by cells with physiological properties identical to unstained cells. Hence, the earliest stage in apoptosis may occur before overt changes in baseline physiology involving the whole cell. Increasing levels of"apoptotic" fluorescence intensity correlate with parallel changes in both membrane potential, which decreases, and cCa, which increases. At later stages of neurogenesis, smaller fractions of cells are apoptotic, many of which continue to be among those actively proliferating, though some of which are in different stages of the neuronal lineage. Independent measures of apoptosis will be required to verify these results since membrane trauma and/or turnover independent of apoptosis might bind annexin V. In vitro studies into the physiological basis of cortical cell migration have continued in the rat and recently in the mouse. Microdissection of rat cortex into VZ/SVZ and CP/SP subpopulations followed by chemotaxis assay has extended previous discoveries: only VZ/SVZ cells respond to femto molar GABA, while both respond to micromolar GABA. FM GABA triggers Ca-dependent directed migration and micromolar GABA stimulates Ca- dependent cell motility independent of a chemical gradient. In the mouse, glutamate rather than GABA triggers these migration and motility responses of cortical cells in vitro. The chemotropic responses to GABA involve pertussis-sensitive signal transduction postnatal ferret whose delayed development permits studying events that occur in utero in rat and mouse.